When the Soviet Union invaded Poland in September 1939, thousands of soldiers were captured by the Red Army. Many Poles surrendered to the Soviets to avoid falling into German hands. Stalin ordered 15,000 Polish officers imprisoned in special camps at Koziels, Ostaszkow, and Starobielsk. He feared these men might form the nucleus for a movement to resist the Communist takeover of Poland in the future.
In April 1940, Stalin ordered 5,000 Polish officers taken into the Katyn Forest, a wooded area near Gneizdovo village, a short distance from Smolensk in Russia. The Poles were marched into the woods, still wearing their uniforms, many with their hands tied behind their backs, and shot in the back of the neck. Thousands were buried in mass graves.
In 1943, the Nazis exhumed the Polish dead and blamed the Soviets. In 1944, having retaken the Katyn area from the Nazis, the Soviets exhumed the Polish dead again and blamed the Nazis.
The Soviets kept this crime a secret. Not until 1989 were Communist documents released documenting the fact that the Soviet NKVD (Narodny Kommisariat Vnutrennikh Del — the secret police force that later became the KGB) carried out the Katyn Massacre.
Sources: Mitchell G. Bard, The Complete Idiot's Guide to World War II. NY: MacMillan, 1998.